A number of approaches to the manufacture of HBTs have been proposed. In particular, US2005/0218399 (Philips) proposes a method of fabricated a SiGeC HBT. In the approach described, as illustrated in FIG. 1, shallow trench isolation (STI) structures 104 are provided on either side of a collector drift region 102 that will form part of the collector. A stack 106 is then formed over the STI structures 104 overlapping region 102, and etched to define a base window or opening.
A layer 108 is then grown in the base window using a non-selective epitaxial process that grows epitaxial SiGeC 110 above the collector region and poly SiGeC 112 on the sidewalls and base of the base window.
After several further process steps, forming an insulator 116, a silicon emitter 114 is formed within the opening, resulting in the stage shown in FIG. 1.
This process has the advantage that the fabrication process can provide a cost-effective solution compared with high performance HBT approaches.
However, there remains a desire for one or more of a further reduction in the size of HBTs, the ease of HBT manufacture, and also the enhancement the figures of merit of an HBT, such as maximum oscillation frequency, bandwidth, and minimum noise. These figures of merit are related to properties of the HBT, including the collector-base capacitance and total base resistance.